This invention relates generally to electronic reference circuits and, more particularly, to a low voltage reference source capable of providing an output voltage which is insensitive to temperature variations and to variations in the power supplied to the reference circuit. The output voltage can be lower than the silicon band-gap voltage. The reference circuit can also be adapted in accordance with this invention to provide a current reference.
Many circuits exist for providing voltage regulation or voltage references. However, many of these prior art circuits suffer from poor temperature compensation and are therefore not suitable for use in logic circuits requiring closely regulated power sources. In a solid state voltage regulator not having temperature compensation, the output will tend to vary with changes in ambient temperature. In most cases, the output variation is due to the negative temperature coefficient associated with the base-to-emitter junction voltage of transistors used in the regulator. As the temperature increases, the base-to-emitter voltage drop, V.sub.BE, decreases and conversely with a decrease in temperature the V.sub.BE increases. In the past, voltage references or regulators have been made temperature independent by combining a positive temperature coefficient component with the negative temperature coefficient voltage of a base-to-emitter junction so that the two different temperature coefficients cancel each other out and, therefore, provide an output that is independent of temperature.
One scheme for obtaining a positive temperature coefficient signal is by taking the difference of the emitter-to-base voltage from two different transistors that are operating at two different current levels. Such a scheme was used on the regulator in U.S. Pat. No. 3,617,859, which was issued to Robert C. Dobkin, et al. The reference provided in the aforementioned U.S. Patent is commonly called a band-gap reference since the reference voltage provided is equal to the band-gap energy of the emitter-to-base junction voltage, V.sub.GO. In order to achieve lower voltages than V.sub.GO the output of the reference must be attenuated such as by a voltage divider network. Then the attenuated voltage is usually buffered by a unity gain-amplifier. However, such a scheme is cumbersome and requires additional circuitry to attenuate and to isolate the reference voltage.
Due to the tight logic level tolerances on many state-of-the-art logic circuits it is desirable to have a low voltage that is independent of temperature and other variations. This is more so the case when low voltage power supplies are used as a voltage source for the logic circuit, e.g., in portable equipment which is powered by dry cell batteries.
Accordingly, one of the objects of the present invention is to provide an output voltage or current having a zero or adjustable temperature coefficient at a voltage less than the silicon band-gap voltage.
Another object of the invention is to provide a reference voltage or a reference current which is insensitive to temperature and to variations in the main power supply and yet produces an output voltage less than the silicon band-gap voltage.